G protein-coupled receptors (GPCRs) constitute a family of proteins sharing a common structural organization characterized by an extracellular N-terminal end, seven hydrophobic alpha helices putatively constituting transmembrane domains and an intracellular C-terminal domain. GPCRs bind a wide variety of ligands that trigger intracellular signals through the activation of transducing G proteins (Caron, et al., Rec. Prog. Horm. Res. 48:277–290 (1993); Freedman et al., Rec. Prog. Horm. Res. 51:319–353 (1996)).
More than 300 GPCRs have been cloned thus far and it is generally assumed that there exist well over 1000 such receptors. Mechanistically, approximately 50–60% of all clinically relevant drugs act by modulating the functions of various GPCRs (Cudermann, et al., J. Mol. Med. 73:51–63 (1995)). Of particular interest are receptors located in dorsal root ganglia. This region of the central nervous system is densely innervated with primary or afferent sensory neurons involved in the transmission, modulation and sensation of pain. Thus, receptors from this region may be used in assays for the identification of new agents for anesthesia and analgesia